It has become recognized that pulsed power supplies employing magnetic compression circuits or modulators, are appropriate for use with high power applications such as, for example, pulsed lasers. Even though such power supplies are more expensive and more complex than many other known types of modulators, the high reliability found in modulators employing magnetic switches makes them well suited for critical applications. Furthermore, the improvement in reliability of magnetic switch modulators over conventional electronic pulsed power supply devices using thyratrons, or the like, is especially prevalent in applications which require very high power output. One example of such a magnetic compression circuit, particularly adapted for high efficiency operation as a laser driving circuit, is found in U.S. Pat. No. 5,177,754 issued to Ball et al.
Traditionally, low power modulators have relied on forced air cooling methods. As power levels increase, the limits of air cooling are quickly reached. High average power magnetic compression circuits generate a great deal of heat, and produce very high voltages. Therefore, prior art high average power magnetic compression circuits have been designed to be operated immersed in a liquid dielectric fluid such as, for example, fluorocarbon dielectric fluids. These dielectric fluids both improve voltage hold off and act as a coolant for the high average power magnetic compression circuits. However, known fluorocarbon dielectric fluids present environmental problems and, as a result, are being removed from the market. Additionally, many prior art high average power magnetic compression circuits discharge waste heat into the surrounding air. Such a discharge of waste heat can adversely affect surrounding components such as, for example, finely tuned optical components. Therefore, it is desirable to produce a high average power magnetic compression circuit which can operate without the use of such fluorocarbon dielectric fluids and which does not discharge significant amounts of waste heat into the vicinity immediately surrounding the high average power magnetic compression circuit. However, previous efforts to produce a reliable device which can provide the necessary power for high power applications, without requiring the use of fluorocarbon dielectric and coolant fluid have been less than completely successful.
Attempts to create such a device have been made using primary switching stages which employ thyratron switches. A thyratron is a gas-filled, hot-cathode electron tube designed for operation at high voltages, high repetition rates, and short time duration conduction pulses. Thyratrons were originally developed for radar modulator applications. However, as additional devices were developed to expand their operational range, that is their peak and average power, repetition rate, current rate of rise (dI/dt) capability, maximum operating voltage, etc., thyratrons found use in a variety of modulator applications. Unfortunately, due to the inherently finite life of such thyratron switches such devices have proven to be less than optimal. Solid state devices have also been tried but these devices generally lack the ability to deliver the instantaneous power and/or rate of change of current which is required.
Early modulators designed for driving copper lasers were based on capacitor inversion circuits. Modulators of this type are still often used in non critical short term experiments. Capacitor inversion circuits are designed to deliver to the laser a voltage pulse having a 40-60 nanosecond (ns) risetime, a duration of 150-200 ns, and a peak amplitude of 20-40 kilovolts (kV). The main commutation switch for these circuits was usually a thyratron.
At an average power level of approximately 10 kW, the peak current and dI/dt of a thyratron in a capacitor inversion circuit (conduction time of 40-100 ns) is extremely high and results in accelerated cathode depletion, gas cleanup, and anode erosion. Thus, as the average input power of modulators approaches the 10 kW level, thyratron lifetimes become too short to perform meaningful long-term experiments. This short thyratron lifetime can be partially alleviated by electrically connecting a non-linear switching element, a magnetic assist, in series with the thyratron. The magnetic assist functions to delay the onset of electrical current for a short time, usually less than 100 ns, while the thyratron is in the formative turn-on process.
To further reduce the electrical stress on the thyratron, a single stage of magnetic compression is often added to a thyratron commutated circuit. In such a circuit implementation, the value of the peak current and dI/dt is linearly reduced by the gain of the magnetic switch. The gain of the magnetic switch is defined as the ratio of the hold-off time to the time required to transfer energy through the switch after saturation of the magnetic core. The functioning of this type circuit is set forth in detail in U.S. Pat. No. 5,189,678 entitled "Coupling Apparatus for a Metal Vapor Laser" to D. G. Ball and J. L. Miller.
At higher average power levels, thyratron commutated multi-stage magnetic compression circuits can be used. However, these circuits, like the prior art compression circuits, are usually immersed in a liquid dielectric/coolant to handle the heat load of the components and prevent electrical breakdown due to high voltage. Such thyratron commutated multi-stage magnetic compression circuits are used, for example, in the Laser Demonstration Facility (LDF) of the Atomic Vapor Laser Isotope Separation (AVLIS) Program at Lawrence Livermore National Laboratory. The use of such non-linear magnetic devices substantially reduces the peak current and dI/dt on the thyratron, and results in an increase in the useful life by extending the conduction pulse to a 1 .mu.s duration. Mechanically, these circuits may be implemented in several ways as shown, for example, in U.S. Pat. No. 5,177,754 entitled "Magnetic Compression Laser Driving Circuit" to D. G. Ball et al. Even with the extended conduction time provided with magnetic compression circuits, operational experience indicates that the thyratron is still the major lifetime limiting component. Eventually, after examination of many switch options, a thyratron replacement which consists of a solid-state assembly fabricated as a series connected stack of 22 asymmetrical thyristors is employed in modulators used by the LDF at Lawrence Livermore Laboratory. This solid-state assembly is a very reliable and cost effective drop-in replacement for the thyratron.
A series stack of thyristors is an expensive assembly, so circuit designs which incorporate single solid state devices or an assembly of devices electrically connected in parallel are also used. Magnetic modulators of this circuit topology operate from low voltage power supplies, less than 1000 volts, and the high voltage required by the load is provided by a high-ratio stop-up transformer located in the magnetic modulator circuit. The conduction time of the thyristors is increased from the approximate 1 .mu.s conduction time used in the LDF modulators to 5-10 .mu.s in order to take advantage of the high average current capability of the solid state devices (i.e. silicon controlled rectifier (SCR), insulated gate bipolar transistor (IGBT), MOS controlled thyristor (MCT), etc . . . ) operating at the longer conduction times. Average power levels of modulators having this general topology range from 3 kW to approximately 100 kW. As with previous magnetic compression circuits, these designs are usually immersed in environmentally unacceptable CFC dielectric/coolants and require large, expensive, and heavy enclosures to contain both the circuit and the liquid dielectric/coolant.
Therefore a need exists for a compact, reliable, long life, high average power magnetic compression circuit, modulator, which can deliver high voltage pulses of short duration at a high repetition rate and high peak power output which does not require the use of environmentally unacceptable fluids such as chlorofluorocarbons either as a dielectric or as a coolant, and which discharges very little waste heat into the surrounding air.